What Is Gravity, Really?

By DENNIS OVERBYE

Published: November 11, 2003

Correction Appended

''Gravity . . . it's not just a good idea. It's the Law,'' reads a popular bumper sticker.

Gravity is our oldest and most familiar enemy, the force we feel in our bones, the force that will eventually bury us, sagging our organs and pulling us down, but for all its intimacy, it is a mystery. What really is the law?

For most of us it's the one that Isaac Newton proclaimed in 1687 as the rule of the cosmos, describing how (but not why) two objects attract each other with a force proportional to their masses and inversely proportional to the distance between them. But it's been rewritten and physicists expect that it will be rewritten again.

Newton's gravity was replaced by Einstein's general theory of relativity early in the 20th century. Einstein suggested that matter and energy warp space-time the way a heavy sleeper sags a cheap mattress, causing planets, basketballs and light beams to move in curved paths instead of straight lines.

General relativity predicted the bending of light, the expansion of the universe and black holes, and has served as the foundation for modern cosmology, but theorists have never presumed that it would be the last word on gravity.

For one thing, it is mathematically incompatible with the quantum laws that govern subatomic particles. In order to describe what happens at very small distances or very high energies corresponding to the first moments after the Big Bang, where space and time become discontinuous, general relativity has to be merged with quantum theory, a project that has engrossed the present generation of physicists.

But recently some experts have been wondering out loud if it is time to rewrite Einstein's version of the law as it applies to the other end of the length scale, to very long distances. The motivation comes from the predominance of what is sometimes called ''the dark sector'' in the universe.

According to what has recently become a highly celebrated ''standard model,'' ordinary atoms make up only 5 percent of the ''stuff'' of the cosmos. Some kind of mysterious dark matter, perhaps consisting of elementary particles left over from the Big Bang, makes up 25 percent, while the rest -- a whopping 70 percent -- consists of something even more mysterious, known as ''dark energy.''

Obviously a theory that leaves 95 percent of the universe unexplained is less than a complete triumph.

Neither dark energy nor dark matter has been observed or detected directly. Each has been inferred from its gravitational effects on the tiny fraction of stuff we can see. As a result, some scientists have suggested that what astronomers have discovered in the last 20 years is their own ignorance of gravity.

In particular, the discovery, five years ago, that the expansion of the universe is apparently accelerating, under the influence of that dark energy, has occasioned a re-evaluation of the old certainties.

The simplest explanation for dark energy is something called the cosmological constant, first invented by Einstein, a cosmic repulsion caused by the energy residing in empty space. But attempts to calculate this energy have resulted in numbers 1060 bigger than what astronomers have measured -- so large that the universe would have blown apart before atoms or galaxies could have formed -- causing theorists to throw up their hands.

''I think we are so confused that we should keep an open mind to tinkering with gravity,'' said Dr. Michael Turner, a cosmologist at the University of Chicago.

As a result of all this, physics literature has become peppered with suggestions of ways to change gravity. This fall, given a choice of explanations for dark energy during cosmology workshop at the Kavli Institute for Theoretical Physics in Santa Barbara, Calif., 20 of the 44 participants voted for some variation of ''Einstein was wrong.''

Some of these proposals take their cue from the science-fiction-sounding string theory, the putative theory of everything, which holds out the possibility that our universe might be a 4-dimensional membrane (or ''brane'') in an 11-dimensional space.

Most of the vibrating strings that make up the particles and forces of nature in string theory would be stuck to the brane, like the nap on a rug. But the strings responsible for transmitting gravity would be able to drift away or ''leak'' into the meta-space surrounding the brane as they traveled along it from distant objects, according to a theory set forth in 2000 by Dr. Gia Dvali, Dr. Gregory Gabadadze and Dr. Massimo Porrati of New York University. The effect, they say, would be to make distant galaxies appear as if they were accelerating as they moved away from us.

Also in a stringy vein is Cardassian expansion, named after the villainous race on ''Star Trek,'' and dreamed up by Dr. Katherine Freese and Dr. Matthew Lewis of the University of Michigan. According to their theory, the universe accelerates as a result of other branes tugging on our own. ''One can get an accelerating universe without having any dark energy,'' Dr. Freese said.

Other theorists are going back and modifying general relativity directly, noting that when he wrote it down Einstein chose the simplest possible equations that would carry out his ideas. But more complicated equations might be necessary. That was the approach taken by Dr. Turner and his colleagues, Dr. Sean Carroll and Dr. Vikram Duvvuri of Chicago, and Dr. Mark Trodden of Syracuse. The result was a universe that would speed up as it got bigger and emptier.

That might sound crazy, Dr. Turner said, but not any crazier than the idea 80 years ago that the universe would be expanding.

The model raises as many questions as it answers, but it and others like it are still worth pursuing, Dr. Carroll said.

''Something funny is going on when the universe gets to be 10 billion years old,'' he said, ''and none of our current ideas is standing up and declaring itself to be the right answer, so we have to be bold.''

Photo: Isaac Newton proclaimed his law of gravity as the rule of the cosmos in 1687. (Photo by Granger Collection)

Correction: November 17, 2003, Monday An article in Science Times on Tuesday answering the question ''What Is Gravity, Really?'' misstated part of Newton's law of universal gravitation. The force attracting two masses is inversely proportional to the square of the distance between them, not simply to the distance.